I have bought a used Trek 4300 bike and after a few days I noticed leaking oil from front suspension fork. Oil leaks only from one leg (opposite to disk-brakes). I would like to know more about construction of front-suspension. Is this a serious problem? Is it possible to refill suspension with new oil? What kind of oil is used? How much oil is used?
While this may in fact be a serious problem which needs attention, it can also be normal. The first thing you need to do is define what you mean by leak.
If it is a thin and light coat of oil on on fork stanchion, this is very likely normal.
If it is a puddle, pool, or drip of oil on the fork leg, this is more than likely a problem. It can be a failed fork seal, or a a damaged damping or lockout cartridge.
Both of these can be caused by over compression of the fork. It could also a be a defect in the fork, which should be replaced. Do you have an LBS you trust? If so, take the bike to them and ask.
Be aware that there are other things that can cause this that may not be damage. How do you store the bike? Do you hang it up on the wall by the wheel or set it upside down on the bars and saddle?
Leaving the fork in a position to have oil draining against the seal can allow it to seep through, but that is normal. I know this is a bit ambiguous, but there is no way to give an answer with certainty without seeing the fork. Post a picture of the leak, and we can be more helpful.
The level of oil in a fork is very important to its proper function.
When I maintain my fork, I remove the caps for each stanchion using a socket wrench. You may be able to do the same with your fork and examine if there are any obvious problems with it.
Since this is a new bike I recommend taking it directly back to the point of purchase and asking for help, or replacement of a defective product.
This is a serious problem that has to be fixed. Each fork has its own specification for the type and amount of oil to be used. Usually, the specs require the oil level be accurate to a few ml's as well as no air bubbles or other contaminants in the oil. If you want to do it yourself you should be able to find the specs for your bike's fork online. However, because it is leaking, the leak must be found and corrected.